This analyst thinks that China would gain control of the waters around the Senkakus and pry any Japanese garrisons off by cutting off their supply lines, if nothing else:
Were a battle over the Senkakus/Diaoyus to take place today, I would give China the edge, even though Japan holds the contested real estate and the United States has committed itself to the islands' defense. Geography and force are the main reasons why.
First consider the islands' geographic merits and drawbacks. The great fin de siècle seapower theorist Alfred Thayer Mahan classifies geostrategic assets by their position, strength, and resources. The Senkakus/Diaoyus occupy an awkward position near the southern tip of the undefended Ryukyus chain, closer to Taiwan than to the Japanese main islands and roughly equidistant between Okinawa and the Chinese mainland. The archipelago's natural defenses are so-so at best, owing to its small size and fragmentation into several islets.
The island chain's geography opens up options for a determined attacker. Rather than mount a full-scale assault, PLA occupiers could grab one island, place weaponry on it, and pummel Japanese GSDF sites from there -- seizing the rest over time through salami-slicing tactics. And since the islets offer virtually no natural resources to support garrisons, everything would have to be shipped in by sea or air. If China rules even a pocket of sea and airspace around the islands, it will probably get its way in a test of arms.
While the analyst is right that Japan needs to spend more on defense to defend their territory, I don't think the odds are good for China to win that fight in the air and sea.
Even without directly intervening by pulling triggers, we can do a lot to help Japan win that fight.
Remember, this would be primarily a sea and air fight over a fairly narrow front.
Yes, the campaign area is relatively far from Japan, but given recent Chinese construction, the campaign area isn't that easy for Chinese air power to reach.
China obviously can't use their far larger army against Japan.
And China's navy is large but not that good yet. Japan has a good navy. Further, Japan could use everything they've got in a war while China has to keep their southern fleet free in case Taiwan or the South China Sea witnesses problems while China is engaged over the Senkakus.
China's northern fleet probably has to keep a lot of assets free in case they need to land troops in North Korea on the west coast to keep South Korean forces out of Pyongyang if North Korea goes belly up.
Plus, if I was Japan, I'd deploy my subs in the mainland-Korean peninsula gap to intercept Chinese ships going south. That would also allow the Japanese to assume that any subs spotted around the Senkakus are Chinese and so can be attacked.
So I don't believe China could use all three of their fleets against Japan. The East Sea Fleet would be the main combatant with only limited help from the neighboring fleets.
In the air, Japan is outgunned. But this is a narrow front. Japan's two wings of F-15 fighters will probably hold their own against the Chinese air force which can only deploy a small portion of their air force against Japan over the Senkaku Islands.
China would have the advantage in a war of attrition since they could replace aircraft lost with their many more aircraft outside of the campaign area.
But we could help, too, by shipping Japan replacement F-15s even if we have to take them from our own stocks until we can replenish them. Attrition might not be that effective if we do that.
Further, we seem to have forgotten the old Cold War rule that when nations fight and nuclear powers are on both sides, the war has to be terminated quickly before the threat of nuclear escalation takes place. China has nukes. And we do too and are Japan's ally. I don't think China can afford to risk a long fight any more than Japan can.
So I assume a war would last weeks, at most. That greatly reduces the ability of China to use weight of numbers to wear down Japan, either at sea or in the air.
To further push China to end the fight, I'd have every electronic intelligence asset active sucking up every Chinese transmission while they wage war. We'll learn a lot. China will hate that.
The biggest question will be who controls the islands with ground troops. If China manages to get troops on the islands first, Japan will have problems invading to liberate them. But Japan does have helicopter carriers (called "destroyers") that could carry troops and a small marine force. Ideally, Japan gets forces to the islands first to set up defenses and then puts mines around the islands to keep China from invading them.
Yet the troops on those small islands will be very vulnerable to air attack. I don't think digging in will be much of an option. Pound them enough from the sea and air, and you don't need much of a marine force to move in and mop up. In the end, air and sea control will determine who stands on the islands.
So the war will be fought on a narrow front by Japanese air and naval forces at least a little better than the Chinese and possibly quite a bit better; and not outnumbered very much at all. China will have the depth to replace losses better than Japan can, but I don't think China will get the time to make attrition have an effect on the fight.
When the fighting stops from fear of escalation to direct US intervention and possible nuclear escalation (not from threats but from the simple fact that two nuclear powers are shooting at each other with conventional weapons), it is quite possible that neither side will have all the islands under their control. Even the side on the losing end--likely to be China, I think--could hold a piece of the islands.
That will make sure the next round starts at a much higher level of force as both sides gear up for a possible round two.
That outcome might get Japan to seriously increase their defense spending and think about having their own nuclear deterrent. That has to be a Chinese nightmare.
And all over small rocks.
UPDATE: I added the map that I meant to use initially.